1. Field of the Invention
The present invention relates to a real image type viewfinder system and, more particularly, to a real image type viewfinder system which includes an optical system having the whole size reduced by using an appropriately set prism and which is arranged in such a manner that a viewfinder image (object image) which is formed as an inverted real image by an objective lens is observed as a non-inverted erecting image by means of the appropriately set prism.
2. Description of the Related Art
In the field of viewfinder systems for photographic cameras, video cameras, or the like, various real image type viewfinder systems have heretofore been proposed which are generally arranged in such a manner that a viewfinder image is observed as a real image.
Such a real image type viewfinder system has recently been widely used in cameras of the type having a zoom lens. This is because it is possible to easily reduce the size of the entire optical system of the real image type viewfinder system as compared to a virtual image type viewfinder.
FIG. 5 is a schematic perspective view showing the essential portions of a conventional real image type viewfinder system which uses a Porro prism for providing a non-inverted erecting image.
The viewfinder system shown in FIG. 5 includes an objective lens 101 having a negative lens 102a and a positive lens 102b each of which is arranged to move along the optical axis of the objective lens 101 in accordance with the zooming of a photographic lens (not shown), a field lens 103, a Porro prism 104 for inverting an inverted viewfinder image formed by the objective lens 101 to provide a non-inverted erected image, and an eyepiece lens 105.
The negative lens 102a and the positive lens 102b which constitute the objective lens 101 perform zooming by moving along the optical axis of the objective lens 101 in accordance with the zooming of the photographic lens, as indicated by the arrows shown adjacent to the respective lenses 102a and 102b. Thus, the observing magnification of the viewfinder image is made to vary in correspondence with a photographing magnification which varies in accordance with the zooming of the photographic lens.
The viewfinder image which is formed as an inverted real image by the objective lens 101 is formed in the vicinity of the field lens 103. The inverted real image is reflected by reflection surfaces 104a, 104b, 104c and 104d of the Porro prism 104, in that order, whereby the viewfinder image is inverted from the inverted real image to a non-inverted erect image. The viewfinder image which is provided as the non-inverted erect image is observed through the eyepiece lens 105.
In the case of the above-described real image type viewfinder system which uses the Porro prism 104 for providing the non-inverted erecting image, the Porro prism 104 has an external shape which partially projects in the horizontal and vertical directions as shown in FIG. 6, with the result that the size of the entire viewfinder system increases.
As is known in the art, it is preferable to dispose the objective lens 104 of the viewfinder system in the vicinity of the photographic lens because parallax can be reduced.
It is also desired to dispose the eyepiece lens 105 in a position such that the nose of a photographer does not come into contact with a camera when the photographer looks into the viewfinder thereof. Accordingly, in the design of an ordinary form of camera, it is necessary to arrange the eyepiece lens 105 in such a manner as to project from the camera body in the rearward direction thereof, or to dispose the eyepiece lens 105 in an end portion of the camera body.
However, the former system has the disadvantage that the portability of the camera is impaired, while the latter system has the disadvantage that it is necessary to space the optical axis of the objective lens 101 apart from that of the eyepiece lens 105 in parallax terms.
In other words, it is necessary to design the viewfinder system in such a manner as to reduce the length of the entire objective lens and the vertical size of the viewfinder system and to space the optical axis of the objective lens 101 and that of the eyepiece lens 101 apart from each other to some extent in the horizontal direction. As a result, the complexity of the entire optical system tends to increase.
As another real image type viewfinder system, a secondary image forming type viewfinder system is known. This type of viewfinder system has, however, the problem that the length of the entire objective lens increases and the size of the entire optical system increases.
Japanese Laid-Open Patent Application No. Sho 61-156018 proposes a real image type zoom viewfinder utilizing the secondary image forming type viewfinder system. The proposed real image type viewfinder includes an objective lens made up of a multiplicity of lens units in such a manner as to perform zooming by causing a predetermined lens unit of the multiplicity of lens units to move along the optical axis of the viewfinder, and a Porro prism is disposed rearward of the objective lens as image inverting means to provide an non-inverted erect image.
In the real image type zoom viewfinder which is suitably used as a viewfinder system in a viewfinder device, if a reflecting member, such as the Porro prism, is used as the image inverting means, it is preferable to utilize total reflection in terms of reflecting efficiency since no evaporated substance (aluminum or silver) is needed.
However, in the system utilizing total reflection, a bundle of rays which does not satisfy a total-reflection condition may pass through the reflection surfaces of the reflecting member and part of the passing bundle of rays may reach an eyepiece lens as stray light to form a ghost or a flare.
FIGS. 14(A) and 14(B) show the state in which the bundle of rays does not satisfy the total-reflection condition, and schematically show the states of the lens arrangement of a real image type viewfinder system in the case of wide-angle-end setting and in the case of telephoto-end setting, respectively. In both figures, a Porro prism is shown in expanded form.
The real image type viewfinder system shown in FIGS. 14(A) and 14(B) includes a first lens unit 91 of negative refractive power and a second lens unit 92 of positive refractive power, and the first lens unit 91 and the second lens unit 92 constitute an objective lens 100. The real image type viewfinder system shown in FIGS. 14(A) and 14(B) also includes a stop 99 fixed to the first lens unit 91, a stop 90 fixed to the second lens unit 92, prism blocks 96 and 97, shown in expanded form, each of which has reflection surfaces as image inverting means, a field-of-view frame 98 disposed in the vicinity of the image forming plane of the objective lens 100 for limiting the field of view of the viewfinder system, and an eyepiece lens 95.
The real image type viewfinder system shown in FIGS. 14(A) and 14(B) performs zooming by causing the first and second lens units 91 and 92 of the objective lens 100 to move along the optical axis thereof together with the stops 99 and 90 as indicated by the arrows shown adjacent to the respective lens units 91 and 92. A viewfinder image formed in the vicinity of the field-of-view frame 98 by the objective lens 100 is conducted to the eyepiece lens 95 via the prism blocks 96 and 97 which serve as image inverting means, so that the viewfinder image is observed through the eyepiece lens 95.
In the above-described zooming operation, it is to be noted that the bundle width of an on-axial bundle of rays for the wide-angle-end setting shown in FIG. 14(A) is greater than that of the on-axial bundle of rays for the telephoto-end setting shown in FIG. 14(B). Accordingly, in the case of the wide-angle end setting, part of the bundle of rays does not satisfy the total-reflection condition on total-reflection surfaces provided in the prism blocks 96 and 97 and passes through the total-reflection surfaces without being totally reflected, to form stray light. Part of the stray light enters the eyepiece lens 95 and impairs the observation condition of the viewfinder image.